Data acquisition systems, such as analog to digital converters (ADCs), normally include a front-end sample and hold stage for capturing an input signal. Typically, this sample and hold stage is implemented with a switched-capacitor circuit in which a sampling capacitor is switched between sampling and integrating modes. Generally, during the sampling mode, the input signal is sampled onto the sampling capacitor and during the integration phase, the charge on the sampling capacitor is transferred to an integrating capacitor.
In order to maintain a high dynamic range, the sampling capacitor in switched-capacitor circuits, such as sample and hold stages, must be large to minimize
  KT  Cnoise. Therefore, high dynamic range sample and hold circuits require that the input signal source be capable of delivering a relatively large current for rapidly charging the large sampling capacitor. In high sampling rate applications, such as delta sigma ADCs, this current requirement becomes even more severe due to the relatively small amount of time available during each sampling event to charge the sampling capacitor. Furthermore, the sampling current is often nonlinear due to charge injection and nonlinear input impedances caused by the switching circuitry controlling the charging of the sampling capacitor, which in turn mandates severe linearity requirements on the input signal source. Finally, the sampling current must settle quickly to avoid distortion as a sequence of samples of the input signal are taken.
In sum, new techniques are required for use in high dynamic range data acquisition systems. These techniques should address the problems related to the use of large sampling capacitors, especially at high sampling rates.